Well, I did some things for Scar and he promised that he would meet me outside the Flaming Fist Barracks after he'd done some paperwork.

I assume the Flaming Fist barracks is that octagonal edifice in the southwest portion of Baldur's Gate. Days pass, but Scar fails to show up anywhere near that building.

Did I do anything wrong or was I victimized by some kind of bug.

Check inside ;)

Or, if you have just arrived back from Candlekeep, he won't be there anyway (not spoiling)

I know what you mean, he does sometimes vanish and I don't know where he is. If you've looked inside and outside the building, at day and night, then it beats me where he is. Good job his quest isn't vital, just a little XP thats all.